The present invention relates to an image forming apparatus of a single pass color system (hereinafter referred to as SPC) in which color forming processes for plural colors are conducted while an image carrier makes one turn, and in particular, to an image forming apparatus wherein plural images each being of a different color can be superposed without positional deviation.
As an image forming apparatus of an SPC system in which the number of copies per a unit of time is increased, there has been proposed, for example, an image forming apparatus wherein four sets (equivalent to four colors) each being composed of a scorotron charger, a scanning optical system and a developing unit which are arranged at prescribed intervals around an image carrier (a photoreceptor drum or a photoreceptor belt) in its rotational direction are arranged, and a transfer unit, a separating unit, a cleaning unit and a fixing unit are further provided so that image forming processes for plural colors may be conducted while the image carrier makes one turn.
In the image forming apparatus of an SPC system wherein plural scanning optical systems are arranged around an image carrier in its rotational direction, it is necessary to prevent deviation in the main scanning direction or in the sub-scanning direction by correcting with various sensors mounted around the image carrier.
However, there are deviation in the mounting positions for various sensors and dispersion in exposure width in the main scanning direction for each color, and these are further subjected to the change with time. As a result, it has been impossible to prevent positional deviation in the main scanning direction or in the sub-scanning direction among plural images each being of a different color.